A fossil-fired steam generator generates superheated steam with the aid of heat created by the combustion of fossil fuels. Fossil-fired steam generators are mostly used in steam power stations which primarily serve to generate electricity. In such power stations the steam is supplied to a steam turbine.
Like the various pressure stages of a steam turbine, the fossil-fired steam generator also comprises a plurality of pressure stages with different thermal states of the water-steam mixture contained therein in each case. In the first (high) pressure stage the flow medium on its flow path initially flows through economizers which use residual heat to preheat the flow medium, and subsequently flows through various stages of evaporator and superheater heating surfaces. The flow medium is evaporated in the evaporator, then any possible residual moisture is separated off in a separation device and remaining steam contained therein is heated up further in the superheater. The superheated steam then flows into the high-pressure part of the steam turbine, is evaporated there and supplied to the following pressure stage of the steam generator. There it is superheated once more and supplied to the next pressure section of the steam turbine.
As a result of a wide diversity of external influences the heating power transferred to the superheaters can fluctuate greatly. It is therefore frequently necessary to regulate the superheating temperature. Usually this is mostly achieved both in the high-pressure stage and also in the medium-pressure stages for intermediate superheating by an injection of feed water upstream or downstream of individual superheater surfaces for cooling, i.e. an overflow line branches off from the main flow of the flow medium and leads to injection valves disposed accordingly there. The injection in such cases is usually regulated via the temperature deviation from a predetermined nominal temperature value at the outlet of the superheater of the respective pressure stage.
Modern power plants not only demand high levels of efficiency but also a method of operation that is as flexible as possible. As well as short startup times and high load change speeds, these also include the option of compensating for frequency faults in the electricity grid. In order to fulfill these requirements the power plant must be able to provide additional power of for example 5% and more within a few seconds.
Such changes in power of a power station block within seconds are only possible by a coordinated interaction of steam generator and steam turbine. The contribution that the fossil-fired steam generator can make to this process is the use of its boilers, i.e. the steam boiler but also the fuel boiler, as well as rapid changes to the adjustment variables feed water, injection water, fuel and air.
This can be done for example by opening partly-throttled turbine valves of the steam turbine or what is referred to as a step valve, through which the steam pressure upstream of the steam turbine is reduced. Steam from the steam boiler of the upstream fossil-fired steam generator is stored by this process and supplied to the steam turbine. With this measure an increase in power is achieved within a few seconds.
A permanent throttling of the turbine valves to maintain a reserve however always leads to a loss of efficiency, so that to drive the system cost-effectively, the degree of throttling should be kept as low as is absolutely necessary. In addition a number of designs of fossil-fired steam generators, for example once-through steam generators under some circumstances demand a significantly lower boiler volume than for example natural boiler steam generators. The difference in the size of the boiler has an influence in the method described above on the behavior during changes in power of the power station block.